A concave surface or curve.
"The bottom of the bowl has a smooth, concave surface that holds the soup perfectly."
To render concave, or increase the degree of concavity.
"The dentist used a special tool to carefully concave the inner surface of my tooth filling so it would fit more smoothly against the gum line."
curved like the inner surface of a sphere or bowl
"The concave shape of the spoon's interior made it perfect for holding soup without spilling."